Development of Standard for Lifting Loops in Precast Deck Beams

摘要

Prestressing strands, bent into loops and cast within beams, are the most common anchorage system used for handling precast beams. No national guidance exists for the design of lifting loops in members that are shallower than 24 inches. To address this shortcoming, the 2003 IDOT Prestressed Concrete Manual provided requirements for lifting loops that were derived from experience and best engineering judgment. An experimental research program was conducted at the University of Illinois to investigate the performance and capacity of lifting loops cast in deck beams, and also on the rupture strength of strand loops that protrude from heavily confined specimens. The key variables in the deck beam tests were the shape of the lifting loop, depth of embedment, side edge distance, number of strands per loop, number of lifting loops in a corner of a beam, and angle of pull. The key variables in the strand rupture tests were the number of strands per lifting loop, the uniformity of strand shape, the constraint of strands by a conduit, and the shape of the contact surface of the lifting device. The results of these experiments, in conjunction with a review of previous work and field practices, support and help justify the recent changes to IDOT lifting loop design requirements as presented in the 2007 ABD Memorandum 07.2. In the absence of another suitable design, a lifting loop must satisfy the IDOT requirements of a minimum 60-degree lift angle, a minimum 6-inch edge clearance, and an embedment depth that is at least equal to the overall depth of the member less 4 inches. The test results demonstrated the benefit of using multiple stands per lifting loop, multiple lifting loops in a corner of a beam, higher lift angles, and conduit to help ensure more uniform shape and engagement of strands. The results of this research have reinforced IDOTs current design requirements for lifting loops and revealed the need for a national standard.